Once you have exported a movie, all edits become embedde, including titles, so they can't be changed or deleted. Go back to your original project and remove the title there. Then you can re-export.
Alternatively, you can import the previously exported movie into a new iMovie project and possibly do some limited editing to remove the title. If the title is overlaid onto a clip, then you would need to trim the clip to remove the title, or possibly split the clip off (Modify/Split Clip), delete it, and replace it with the original source clip that has no title. If the title appears separately on a black background you can do a Modify/Split clip and then delete the title and background. .
-- Rich